The interstellar medium (hereafter ISM) was first discovered in 1904, with
of stationary calcium absorption lines superimposed on the Doppler shifting
spectrum of a spectroscopic binary. Since the calcium lines were not
changing in wavelength, they could not originate in the stellar
atmospheres of the binary star, and so had to be between the telescope and
the star. Since no terrestrial source was identified, the calcium had to be
interstellar. Several more discoveries have been made since then: the
discoveries of interstellar extinction (the dimming of starlight) in the
1930's, of polarization by interstellar dust grains in 1949, of
21-centimeter radio emission from atomic hydrogen in 1952, of the soft
X-Ray background in the 1960's, and many others.
The ISM is now known to consist of several independent phases, including:
- hot, ionized medium (HIM)
- warm, ionized medium (WIM), also called diffuse ionized gas (DIG)
- warm, neutral medium (WNM)
- cold, neutral medium (CNM), and
In addition, several other phases are observed and classified separately,
including HII regions (the "II" is a Roman numeral two,
indicating singly-ionized hydrogen), supernova
hydrogen clouds, and molecular clouds. Here's a brief description of each.
Hot, ionized medium
The HIM consists of very hot gas at temperatures higher than half a million
kelvins. This gas is heated by supernovae explosions which blast
large bubbles of hot gas into the surrounding interstellar medium. The gas is
very thin -- less than 10-4 atoms per cubic centimeter -- but it fills
nearly half the volume of a typical spiral galaxy. It is detectable in X-rays,
and by the absorption lines of highly-ionized atoms (oxygen VI for example).
Warm, ionized medium
The WIM is cooler, with temperatures of a few tens of thousands of kelvins.
Gas densities are higher than in the HIM, though it takes up only 20 percent
of the volume. The WIM is interesting because a lot of energy is required to
keep it ionized in a steady state. Part of this energy is supplied by massive,
luminous stars (type O and B). It is easily detectable by
observing galaxies at the wavelengths of ionized hydrogen, and takes up
20 to 40 percent of the total hydrogen Balmer line emission of a given
galaxy. The WIM can also be detected by measuring the phase shift of pulsars
with frequency caused by the effect of ionized plasmas on radio waves.
Warm, neutral medium
The WNM is cooler and denser still, with temperatures less than 10,000 kelvins.
It takes up about 20 to 25 percent of the volume in a galaxy. Since the
hydrogen gas is neutral (the ionization temperature is over 11,000 kelvins),
we can detect it in the neutral hydrogen radio emission line at 21 centimeters
(about 1.4 gigaHertz), and also in the absorption lines of weakly ionized
metals (for example, the calcium ions mentioned in the first paragraph).
Cold, neutral medium
The CNM is made up of cooler hydrogen clouds at
temperatures less than a
few hundred kelvins, and molecular clouds containing a wide variety of
molecules (H2, CO, water, and organic molecules) at very cold
temperatures -- between 3 and 50 kelvins. The temperature can never drop below that of the cosmic microwave background at about 2.7 kelvins. This gas takes up only a small
fraction of the volume in a spiral galaxy, but contains most of the mass of the gaseous ISM. This gas can be
detected in radio and microwave emission. It is important to note that
neutral hydrogen (both in the WNM and CNM) is an excellent absorber of
of soft X-Rays with wavelengths longer than one angstrom. However, molecular
hydrogen (H2) does not emit radiation (since the molecule is
symmetric, it has no dipole to generate the radiation), so carbon monoxide
(CO) is often used as a tracer to measure the amount of H2 in a given cloud.
Dust grains can be found scattered throughout the galaxy, as long as the
temperature is not high enough (several thousand kelvins) to evaporate the grains. The dust acts to diminish and redden
starlight, and is detectable primarily in this way. It can also emit
infrared light of its own, and can be seen with microwave and long-wavelength
infrared telescopes (for example the IRAS satellite). Dust is
responsible for the pronounced lack of background galaxies seen along the
plane of the Milky Way -- their light is absorbed by dust grains in the
plane of our Galaxy, known as the zone of avoidance. Dust also makes it
difficult to see into the core of our own Galaxy, and is responsible for
such sights as the Horsehead Nebula and the Coal Sack. It is also
responsible for the blue haze surrounding the Pleiades, which arises because
dust grains preferentially scatter blue light from stars in the cluster.
Supernova remnants like the Crab Nebula and HII regions
like the Orion Nebula are also
considered a part of the ISM, and make up most of the more spectacular
visible examples of it. Cosmic rays are also considered a part of the
ISM and are responsible for ionizing some of the colder gas. Finally,
magnetic fields also permeate the Galaxy, and provide some pressure support
via the magnetic pressure, B2/(8*pi).
The ISM is different depending upon the type and environment of the galaxy
you are observing. The phases mentioned here can be seen in most
gas-rich spiral galaxies like our own Milky Way. Gas-poor
elliptical galaxies, or spirals in galaxy clusters
have already used up most of their gas to form stars, or have had it stripped
away by ram pressure from the intracluster medium.